1. Field of the Invention
The present invention relates to a following distance control device and a following distance control method which control an own vehicle on the basis of the following distance between the own vehicle and a preceding vehicle.
2. Description of Related Art
Examples of a following distance control device that controls an own vehicle on the basis of the following distance between the own vehicle and a preceding vehicle include one described in Japanese Patent Application Publication No. 2001-225669 (JP-A-2001-225669). According to this following distance control device, the following distance between the preceding vehicle and the own vehicle is measured by a forward looking sensor such as a laser radar or a millimeter-wave radar, and a following time obtained by dividing the following distance by the vehicle speed of the own vehicle is controlled so as to be a target time or more, thereby keeping the following distance between the preceding vehicle and the own vehicle at a target following distance or more. In a case where a following distance control is performed across all vehicle speeds in such a following distance control device, when the own vehicle is traveling at a predetermined speed or less, in conditions such as congestion and the preceding vehicle stops, a control is performed to bring the own vehicle to a stop in order to keep the following distance.
However, such a following distance control device uses the wheel speed measured by a wheel speed sensor to find the vehicle speed of the own vehicle. Since this wheel speed sensor cannot detect a wheel speed in an extremely low speed range, for example, 1 km/h or less, in that extremely low speed range, the wheel speed and hence the vehicle speed become zero even when the own vehicle has not stopped.
Therefore, the determination as to whether the own vehicle is at a stop is made on the basis of the time elapsed after the wheel speed becomes zero, and the base requested acceleration for the own vehicle is continued to be computed on the basis of the following distance between the preceding vehicle and the own vehicle even during the period of this elapsed time. If the preceding vehicle accelerates at this time, the following distance increases, and so does the relative speed between the preceding vehicle and the own vehicle. Accordingly, the base requested acceleration also increases, causing the own vehicle to accelerate.
If, thereafter, the determination of the stop of the own vehicle is made, and the minimum value of the requested acceleration for stopping is computed and a stop-holding brake is applied to the own vehicle, this means that the stop-holding brake is applied after the own vehicle accelerates and thus a sudden brake is applied during acceleration, which produces an unpleasant shock on the own vehicle and makes a smooth stop impossible.